The present invention relates to a method for the start-up of an electroslag remelting furnace system, in particular a system having a bottom pouring device associated with a mold in which at least one consumable electrode is remelted into an ingot, and a slag transfer apparatus which is positionable adjacent to said bottom pouring device and which is operable to control the transfer of molten slag into said device.
In the electroslag remelting of metals, a bath of molten slag is obtained in a remelting zone, for example, a crucible or a mold (often referred to as a crystallizer). At least one consumable electrode is disposed so it depends into the remelting zone with its lowermost end immersed in a molten slag bath. The operating power is electrical, current flowing from the electrode to and through the slag bath which is a resistive conductor. The passage of the current through the molten slag produces heat which causes the electrode to melt. As the end of the electrode melts, its relative position to the mold is shifted to maintain the lower end portion in the molten slag bath until substantially all of the electrode is progressively melted. Because the metal melted from the electrode has a density greater than that of the slag bath, the droplets of molten metal fall through the molten slag and a molten pool of metal is formed below the slag bath. This molten pool of metal progressively solidifies into an ingot of refined metal.
Previously known in the art are various kinds of apparatus for electroslag remelting of metal, using consumable electrodes in a cooled crucible, disposed on a bottom plate and, for carrying out the remelting process, a pool of molten slag is formed in said crucible.
In one previously known system, the molten slag pool is obtained in the furnace crucible by melting of a solid flux or a mixture of its charge constituents during the remelting of a consumable electrode directly in the furnace crucible. In another case, non-consumable electrodes, carbon or graphite, are employed in the furnace crucible for these purposes. In either case this method is known as the "dry start" method.
The previously known art has also employed premelting of flux, or a mixture of its charge constituents, in a separate unit followed by top pouring the molten slag thus obtained into the top open end of the furnace crucible. This method is referred to herein as "top pouring".
In the first two "dry start" cases of preparing the molten slag pool, the time required for obtaining an ingot is increased by as much as 10 to 20 percent over that required with a molten slag start since the melting of slag is carried out directly in the furnace crucible. This of course is likely to decrease the production rate of the plant by as much as 10 to 20 percent.
When preparing the molten slag pool with the use of consumable electrodes, an incomplete melting of the flux can occur in the peripheral zone of the crucible. That condition is likely to drastically impair the surface of the ingot being melted and will increase the bottom discard, to be cropped during the subsequent processing of the ingot, up to 10 percent of the whole ingot.
Although preparation of the molten slag pool in the furnace crucible by top pouring of molten slag is an improvement over the "dry start" methods, and does increase the production rate of the plant and ensures a higher quality of the bottom part of the ingot, the top pouring method has disadvantages. For example, when placing the consumable electrode in the crucible, the gap between electrode and crucible side walls is small, and the pouring of the molten slag presents difficulties. As it is poured from the top, molten slag gets on the crucible walls and on the consumable electrodes and can and does short circuit the system. Short circuits cannot be tolerated so top pouring is done with power turned off. It also can produce slag sows or lumps thereon. The falling off or dropping of the slag sows into the slag pool during the melting process may result in marked variations of electrical conditions of the melting process.
To eliminate those disadvantages in top pouring requires that during the pouring of the molten slag the consumable electrode should be outside the crucible, for which reason the design of the plant must provide sufficient height or head room for lifting the electrode clamped in the electrode holder over the crucible so that the latter can be displaced from under the electrode for pouring the slag therein.
The electric circuit is elongated thereby, the lines cannot be kept short, and consequently, the losses of active energy increase therein, which results in a reduction of the power factor of the plant (cos.phi.). In top pouring, after the molten slag is in the crucible, a voltage is applied to the installation, and the consumable electrode is lowered at a maximum speed into the crucible until it is brought into contact with the slag. During the elapsed time, a crust or lining of the solid slag may form on the crucible walls and on the cooled bottom plate or on a dummy bar, if one is placed on the bottom plate, which crust is likely to insulate the molten slag pool from the bottom plate and crucible. An insulating crust results in an open circuit so the melting process may never start. The resultant down time to recondition the furnace for starting is costly not only in lost time but in non-productive labor, and wasted power used to melt the slag.
Disadvantages of the existing plants employed for effecting the electroslag remelting of metal according to the top pouring method, consist in excessive plant height, which is required because of the requirement that pouring of the molten slag is done with the consumable electrode raised. This necessity also results in considerable losses of time required for effecting auxiliary operations. Top pouring is dangerous because the molten slag is raised to a height above the mold for pouring. Besides, dummy bars or sacrificial plates are required for protecting the bottom plate against the burning through which occurs if the consumable electrode is inadvertently lowered too far down through the molten slag and an arc occurs between the electrode and the bottom of the crucible.
A bottom pouring device of the type herein described is disclosed and claimed in U.S. Pat. No. 3,670,089 which is a continuation-in-part of copending applications Ser. No. 592,054, entitled "A Method of Electroslag Remelting of Metal and Plant for Effecting Same" filed Nov. 4, 1966; Ser. No. 10,419 entitled "Method and Apparatus for Electroslag Remelting of Metals" filed Feb. 11, 1970; Ser. No. 10,485 entitled "Slag Introduction Method for Electroslag Remelting Process" filed Feb. 11, 1970, the latter two, in turn, being continuations-in-part of said application Ser. No. 592,054; and Ser. No. 61,014 entitled "Method of Electroslag Remelting of Metal and Plant for Effecting the Same" filed July 9, 1970 as a divisional of application Ser. No. 592,054, all of the afore-identified applications being now abandoned.
The provision of a bottom pouring device on an electroslag furnace mold provides a critical element for the start-up of the furnace but does not provide an improved operational start-up system by which molten slag may be raised to pouring temperature and then transferred into the bottom pouring device so that the pouring can be controllably discontinued at a predetermined point in the furnace operation. While the provision of the bottom pouring device solves many of the prior art disadvantages and problems as set out above, it does not provide the complete improved start-up system and molten slag handling methodology of this invention.